Prior-art control elements for industrial trucks are usually divided into several parts depending on their function. As soon as more than one truck function must be controlled with such control elements, both hands of the operator are required. This makes simultaneous steering of the truck impossible, so that the truck must, in principle, be stopped to operate its functions.
Control elements in which a truck function, e.g., a drive command, is actuated continuously via a grip, and another function of the truck, e.g., the lifting and lowering of the load platform, is induced by actuating pressure switches or rocker-type switches, which are within a close range of and outside the control grip, are already known. This makes it difficult for the operator to operate either the control grip or the pressure switch or rocker-type switch, because the latter often may be outside the action zone of the thumb, for which they are mostly intended, and thus cannot be operated simultaneously when the control grip has been turned. This makes it impossible to superimpose these truck functions or simultaneous movement processes that would result. Aside from this, such devices usually require separate control circuits, whose number matches that of the individual switches. Control elements in which pressure switches or pushbuttons are already arranged directly on the control grip, e.g., on its front side, rule out continuous function control.
This is also true of the pilot control device disclosed in German Utility Patent No. DE-GM 78,22,523, which consists of a T-shaped adjusting lever and a plurality of pushbuttons or selector levers arranged on it, whose operation is cumbersome and which do not rule out operating errors.
In guided-surface industrial trucks, especially rail-borne aisle forklifts, the accident prevention regulations specify that both hands of the operator must be bound to signal transmitters during travel in the aisle between the shelves, so that they cannot be extended beyond the outer contours of the truck into the aisle between the shelves. Circuitry (which stops operation) is known in the art wherein the circuitry forces the operator to keep both hands on the control grips. This has been established by national requirements and has been incorporated into standards such as SAFETY STANDARD FOR LOW LIFT AND HIGH LIFT TRUCKS established by The American Society of Mechanical Engineers (ASME/ANSI b56.1-1988). Use of the standard along with an understanding of the circuitry needed for two hand acknowledgement is known in the art, for example from U.S. Pat. Nos. 4,645,030 and 3,491,867 which are hereby incorporated by reference.
Such a two handed acknowledgement standard leads to the difficulty that due to the forced acknowledgment of the two-hand operation, only one hand is available for issuing actual control commands for horizontal or vertical movement of the truck. As a consequence, the operator performs, for example, the forced acknowledgment for two-hand control with his left hand, while the horizontal travel movements of the industrial truck are performed with the right hand. To initiate the vertical lifting movements, the operator is forced to reach over, at least with the steering hand, to a second, separate control element or a corresponding signal transmitter, which is usually located on the other side of the control panel, which will then obviously require both hands to reach for the other grips, because forced acknowledgment must than be given with the other hand. Play time-optimizing diagonal travels or movements are ruled out with such devices.
In a control element for industrial trucks, which is known from German Offenlegungsschrift No. DE-OS 39,25,872, a multifunction control grip for the continuous actuation of a controller diagram is additionally equipped on its circumference with a separate signal transmitter for the continuous and/or finely stepped actuation of a lifting controller, so that travel control and/or lifting control can be performed simultaneously or in a superimposed manner. Even though this control element offers great advantage for freely traveling industrial trucks, it is unsuitable for guided-surface aisle forklifts because of the lack of forced acknowledgment for two-hand operation.